Apparatus for effecting reliable heat transfer of bare die microelectronic device and method thereof

ABSTRACT

Apparatus and method include using a bare die microelectronic device; a heat sink assembly; a heat sink mounting assembly for mounting the heat sink assembly independently of the bare die microelectronic device; and, a force applying mechanism that compression loads, under controlled forces, a surface of the bare die into a direct heat transfer relationship at a thermal interface with a heat sink assembly.

BACKGROUND OF THE INVENTON

The present invention relates generally to methods and apparatus foreffectively dissipating heat reliably in compact processor componentsthat are particularly adapted for computing systems, high performancegame systems, and other high performance microelectronic applications.

Electronic servers and processors generate significant amounts of heatwhile performing their jobs. Advancing high-density semiconductorsystems have increased the demands placed on their thermal management.Such demands are attributable to requirements for higher power resultingfrom the higher speed microprocessors, integrated circuits, and otherelectronic components.

Processor components include a processor bare die containing themicro-circuitry of the processor. Heat is generated in the processorbare die, largely by the power required to drive high-frequencyoperations. In some cases, almost all of this power is dissipated asheat. With the high degree of power generated in more advanced chips,significant heat issues arise. In some cases, processors dissipate over280 watts of power in a relatively small space and overheat. Forexample, as the processor bare die overheats, the timing characteristicsof signals may change, thereby causing intermittent operation andperhaps failure of the processor. Accordingly, successful heat transferis extremely important for the components and systems to perform asintended.

One known heat transfer approach is for having bare die processorsencapsulated underneath a sturdy heat spreader that is coupled to a heatsink. The heat spreader not only serves to spread the heat, but alsoprotects the bare die processor. However, such spreader plates tend toinhibit heat transfer because there are two thermal interfaces involved;one between the bare die and the heat spreader, and the second betweenthe heat spreader and the heat sink. Attempts to transfer heat directlyfrom the bare die through a single thermal interface have been made.However, the direct mounting of the heat sink on the bare die presentsenormous potential problems insofar as the latter are fragile andrelatively easily breakable.

Accordingly, without the ability to transfer heat successfully andreliably directly from processor bare dies without the latter breakingor fracturing, the potential of highly effective heat transfer may notbe entirely achieved. As such, there is a need to do so in a reliableand economic manner, in order to support high frequency and powerrequirements of the processor chips.

SUMMARY OF THE INVENTION

This invention is related to methods and apparatus for effectivelydissipating heat reliably in compact processor components that areparticularly adapted for computing systems, high performance gamesystems, and other high performance microelectronic applications withoutnegative effect and that overcome many of the disadvantages of priorart.

In an illustrated embodiment, there is provided an apparatus comprising:a bare die microelectronic device; a heat sink assembly, a heat sinkmounting assembly for mounting the heat sink assembly independently ofthe bare die microelectronic device; and, a mechanism that loads asurface of the bare die microelectronic device into a direct heattransfer relationship at a thermal interface with a heat sink assemblyunder controlled forces applied by the mechanism.

In an illustrated embodiment, there is provided a method of transferringheat from a bare die microelectronic device comprising: supporting aheat sink assembly independently of a bare die microelectronic device;and, urging a surface of a bare die microelectronic device into directheat transfer relationship at a thermal interface with the heat sinkassembly under controlled forces applied by the mechanism.

These and other features and aspects of this invention will be morefully understood from the following detailed description of thepreferred embodiments, which should be read in light of the accompanyingdrawings. It should be understood that both the foregoing generalizeddescription and the following detailed description are exemplary, andare not restrictive of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a heat transfer assembly ofthe present invention.

FIG. 2 is an elongated side elevation view of the heat transfer assemblydepicted in FIG. 1.

FIG. 3 is an enlarged side elevation view, partly in cross-section, ofthe bare die microelectronic device on the present invention along witha force applying assembly that urges the bare die microelectronic deviceinto intimate heat transfer engagement with a heat sink assembly.

DETAILED DESCRIPTION

FIGS. 1-3 illustrate one preferred embodiment of a heat transferassembly 10 made in accordance with the principles of the presentinvention. Included in the heat transfer assembly 10 are a heat sinkassembly 12, a heat sink mounting assembly 14, a bare diemicroelectronic device 16, a printed circuit board 18, and a forceapplying assembly 20 that are adapted to be mounted within a computersystem 22 having a housing assembly 24, such as a computer server systemhousing assembly 24.

The bare die microelectronic device 16 includes a semiconductor bare dieor chip assembly 26 containing a semiconductor bare die 28. Thesemiconductor bare die 28 contains the micro-circuitry of themicroelectronic device 16. Heat is primarily generated here largelybecause of the power to drive the high frequency, logic-leveltransitions. In a preferred embodiment, the bare semiconductor bare die28 may be of the type, such as PowerPC® that is commercially availablefrom International Business Machines Corporation, Armonk, N.Y. While thepresent invention is directed towards mounting a heat sink on a baredie, it is not intended to be so limited.

The illustrated heat sink assembly 12 includes a base portion 30 and afin portion 32. The base portion 30 may be comprised of a rectangularprism having sufficient size and thermal conductivity to effectivelyconduct heat from the semiconductor bare die 28 evenly over the entirebase portion. Embodiments envision thermally conductive materials, suchas aluminum or copper. Clearly, other suitable thermally conductivematerials and those yet to be developed materials may be utilized. Thefin portion 32 comprises a plurality of thermally conductive fins 34that transfer heat from the base portion 30 to air inside the computersystem housing assembly 24. In this regard, the thermally conductivefins 34 are ideally made of highly thermally conductive materials, suchas copper. Clearly, other suitable thermally conductive materials andthose yet to be developed may be utilized.

The printed circuit board 18 may have a generally elongated and thinconstruction mounted by suitable structure (not shown) within thehousing assembly 24. The printed circuit board 18 carries the bare diemicroelectronic device 16 and with other components thereon, but theother components do not, per se, form aspects of the present invention.

The heat sink mounting assembly 14 may include mounting posts 40 thatcooperate with openings in lugs 42 formed integrally with the heat sinkassembly base. The mounting posts 40 extend thru openings in the printedcircuit board 18 and are secured threadedly to corresponding ones ofstandoffs 44 supported by a chassis wall structure 46 of the housingassembly 24. By tightening the mounting posts 40, as by rotating them,the heat sink assembly is displaced towards the bare die. Accordingly,the heat sink assembly 12 is mounted and supported directly by thechassis wall structure 46 and not directly on the bare die structure.Significantly, loading issues of the heat sink assembly directly on thebare die are removed from consideration; other than the loading appliedby the force applying assembly 20 to be described hereinafter. This issignificant in terms of minimizing the direct loading on the bare die bythe heat sink assembly. As a result, there is less of a tendency for thebare die to fracture or otherwise become damaged while at the same timeeffecting highly efficient heat transfer. While the mounting posts 40are utilized for applying controlled forces, the present inventioncontemplates other mechanisms for applying forces for urging the baredie microelectronic device towards the heat sink assembly.

Spacers or shims 48 are interposed between the bottom surfaces of thelugs 42 and an upper surface of the printed circuit board 18 surroundingthe mounting posts 40. The spacers or shims 48 are utilized for purposesof maintaining printed circuit board planarity under the loadingconditions of the present invention. The arrangement of the spacer 48serves to minimize undesired stresses at a thermal interface 50 betweenthe heat sink assembly and the bare die microelectronic device. Thematerial selected for the spacers should not relax over time afterinitial compression during loading. Otherwise, any retention forcesprovided by the resilient foam material are reduced with the passage oftime, thereby decreasing actuating load.

In the illustrated embodiment, the force applying assembly 20 serves toload the bare die microelectronic device 16 into thermal engagement withthe heat sink to maintain a uniform thermal interface between thelatter. As a consequence, the bare die 28 may be loaded in a controlledmanner into intimate engagement with the heat sink base portion withsufficient force to maintain a uniform engagement while avoiding beingfractured or otherwise damaged under loading. As a result, only a singlethermal interface is required for affecting the heat transfer in areliable manner.

In an exemplary embodiment, the force applying assembly 20 includesessentially an insulator substrate or pad 52, a stiffener element 54, aspring plate 56, a bushing 58, a foam substrate or pad 60, and anactuating member 62, such as screw 62.

The insulator substrate or pad 52 may be made of a thin elastomericmaterial that is sandwiched between a bottom surface of the printedcircuit board 18 and the spring plate 56. The insulator pad 52 serves toprotect the circuit board 18. The insulator pad 52 has openings forcomponents, such as capacitors, that are mounted on the printed circuitboard. The insulator pad 52 has openings for allowing the mounting poststo pass therethrough. For the insulator pad 52, the present inventionenvisions resiliently deformable materials, such as apolycarbonate-based Lexan® that is commercially available from GeneralElectric. Clearly, other suitable resiliently deformable materials, suchas high-density elastomers and non-conductive metals may be utilized, aswell as those yet to be developed.

The stiffener element 54 may be positioned beneath the insulator pad 52.The actuating member 62 acts directly on it for transferring the loadingforces of the force applying assembly 20. The stiffener element 54 maybe a metallic plate having the size of at least the bare die 28, andbeing made of a suitably rigid yet thermally conductive material, suchas aluminum or steel. Interposed between the insulator pad 52 and thespring plate 56 are spacers 64. The spacers 64 are made of a resilientmaterial that enable them to control deflection of the printed circuitboard in a manner to keep the latter relatively planar during loading.The material selected for the spacers 64, preferably, should not relaxover time after initial compression during loading. Otherwise, theretention forces provided by the resilient foam material are reducedwith the passage of time, thereby decreasing actuating load. This alsominimizes the occurrence of a faulty thermal interface over time. In anexemplary embodiment, the spacers 64 may be made of high density, lowcreep foam or the like. An exemplary embodiment of such foam is Pureon800 made by Rogers Corporation, Minneapolis, Minn. Clearly, otherembodiments envision other kinds of materials.

The spring plate 56 is made of a deflectable plate(s) that provides foradjustable clamping forces. The spring plate 56 biases the stiffenerelement 54 against the insulator pad 52 and the printed circuit board 18when the spring plate 56 is displaced as will be described. In thisregard, a bushing 58 is affixed to the center of the spring plate 56 andis threadedly coupled to the actuating member 62. The spring plate 56will bow upwardly as viewed in the drawings in response to the actuatingmember 62 being rotated in a known manner. The actuating member 62 isthreadedly connected to the computer housing assembly 24 and may beexternally actuated by a user. The spring plate 56 may be actuated toprovide adequate load levels to maintain a firm engagement at thethermal interface 50. In the present embodiment, a loading of aboutthirty-five (35) pounds are used. While a single spring plate and loadactuating device are disclosed, the present invention is not so limited.Other force levels, of course, are envisioned for affecting a good heattransfer relationship at the thermal interface 50 depending on thematerials and circumstances involved.

A resilient foam substrate or pad 60 preferably made of the samematerial as the spacers are interposed between the housing assembly andthe spring plate. The resilient foam substrate or pad 60 supports theloading of the spring plate. The material selected for the resilientfoam substrate or pad 60 should not relax over time after initialcompression during loading. Otherwise, the retention forces provided bythe resilient foam material are reduced with the passage of time,thereby decreasing actuating load. This also minimizes the occurrence ofa faulty thermal interface over time.

Reference is now made to FIG. 3 for illustrating the single thermalinterface 50 between the bare die microelectronic device and the heatsink assembly. A thermal interface material 70 provides a good thermalpath to the heat sink. Many kinds of materials are contemplated for useas the thermal interface material 70, such as a thermal grease 70. Anysuitable thermal grease may be utilized. Alternative embodiments for thethermal interface materials include thermal pads, epoxies, phase changematerials (e.g., waxes or polymers) and elastomers. The thermalinterface materials 70 may include a fine metallic powder with highthermal conductivity. In an exemplary embodiment, the thermal interfacematerial 70 may be 2 mils thick.

A bottom surface of the semiconductor bare die 28 is mounted directly ona ceramic dielectric substrate 72 though a suitable surface mountassembly, such as a ceramic ball grid array assembly 74. The ceramicball grid array assembly 74 may include a fine pitch solder ball grid,such as in the order of about 1574 balls for an area of about 52 mm by52 mm. Other surface mount technology may be used instead of the ballgrid array assembly. Preferably, the present invention envisions the useof adhesive filler material(s) 76 interposed in the ball grid arrayassembly 74. The adhesive filler material 76 serves to inhibit orprevent any grounding of the solder balls when loading forces areapplied to such an extent that the solder balls are undesirablydeformed. Also, the adhesive filler material 76 serves to promote heattransfer by eliminating air voids in the interstices of the ball gridarray assembly. The adhesive filler materials selected may act,preferably, to prevent relaxation of compression forces urging the baredie into engagement with the heat sink at the thermal interface 50.Moreover, the adhesive filler material 76 may also include some metallicpowder, such as alumina. This is done in order to serve as an additionalheat path. In an exemplary embodiment, the adhesive filler materials arepreferably epoxy adhesives. In this embodiment, an epoxy adhesive, suchas Hysol FP 6110 that is commercially available from Loctite Corp. maybe utilized. Other adhesive filler materials are, of course,contemplated.

The ceramic dielectric substrate 72 is mounted, in turn, on a ceramicball grid array assembly 78. The ceramic ball grid array assembly 78 is,in turn, surface mounted on the printed circuit board 18 in a knownmanner. The ceramic ball grid array assembly 78 may be provided with anadhesive filler material(s) 80 that serves as an adhesive; much as inthe manner noted above in regard to the adhesive filler materials 76.The adhesive filler material 80, preferably, acts to prevent relaxationof compression forces urging the bare die into engagement with the heatsink at the thermal interface 50.

Based on the foregoing construction, the effectiveness of the heattransfer operation thereof is self-evident. However, to supplement suchdisclosure, the present invention provides for a method of transferringheat from a bare die microelectronic device to a heat sink assemblycomprising: supporting a heat sink assembly independently of a bare diemicroelectronic device; and, urging a surface of a bare diemicroelectronic device into direct heat transfer relationship at athermal interface with a heat sink assembly under controlled forces. Ina preferred embodiment, the process is enhanced by placing a thermalinterface material at the thermal interface between the bare diemicroelectronic device and heat sink assembly. Moreover, the aforenotedurging includes a force applying mechanism that is actuatable to deliverthe controlled forces to the bare die microelectronic device to urge thelatter into the heat transfer relationship with the heat sink assembly.

The embodiments and examples set forth herein were presented to bestexplain the present invention and its practical applications and tothereby enable those skilled in the art to make and use the invention.However, those skilled in the art will recognize that the foregoingdescription and examples have been presented for the purposes ofillustration and example only. The description set forth is not intendedto be exhaustive or to limit the invention to the precise formsdisclosed. In describing the above preferred embodiments illustrated inthe drawings, specific terminology has been used for the sake ofclarity. However, the invention is not intended to be limited to thespecific terms selected. It is to be understood that each specific termincludes all technical equivalents that operate in a similar manner toaccomplish similar purposes and yet to be developed equivalents. Manymodifications and variations are possible in light of the aboveteachings without departing from the spirit and scope of the appendedclaims.

1. Apparatus comprising: a bare die microelectronic device; a heat sinkassembly, a heat sink mounting assembly for mounting the heat sinkassembly independently of the bare die microelectronic device; and, aforce applying mechanism that loads, under controlled forces, a surfaceof the bare die microelectronic device into a direct heat transferrelationship at a thermal interface with the heat sink assembly.
 2. Theapparatus of claim 1 further comprising: placing a thermal interfacematerial at the thermal interface between the bare die microelectronicdevice and heat sink assembly to transfer heat effectively.
 3. Theapparatus of claim 1 further comprising: a surface mount assembly thatcouples the bare die microelectronic device to a first surface of aprinted board.
 4. The apparatus of claim 3 wherein the force applyingmechanism delivers the controlled forces to a second surface of theprinted board that is opposing to the first surface.
 5. The apparatus ofclaim 3 wherein the surface mount assembly includes a first ball gridarray assembly.
 6. The apparatus of claim 5 further comprising a firstfiller material disposed in the first ball grid array assembly thatserves to fill air voids and adhesively attach the first ball grid arrayassembly to the printed board.
 7. The apparatus of claim 6 wherein thefirst filler material substantially reduces or eliminates relaxationthereof following compression loading of the first ball grid arrayassembly.
 8. The apparatus of claim 5 wherein the bare diemicroelectronic device includes a silicon bare die mounted on adielectric member; a second ball grid array assembly mounts the siliconbare die to the dielectric member; and, a second filler materialdisposed in the second ball grid array assembly that serves to fill airvoids therein, wherein the filler material of the second ball grid arrayassembly substantially reduces or eliminates relaxation thereoffollowing compression loading of the second ball grid array assembly. 9.The apparatus of claim 3 wherein the heat sink mounting assemblyincludes mounting fastening members that secure the heat sink assemblyto a supporting structure, and a plurality of resiliently deformablespacers are interposed between the printed board and the mountingfastening members.
 10. The apparatus of claim 1 wherein the forceapplying mechanism includes at least a spring member that is actuated byan actuating member for applying the controlled forces.
 11. Theapparatus of claim 10 wherein the force applying mechanism includes astiffener element interposed between the actuating member and theprinted board.
 12. A method of transferring heat from a bare diemicroelectronic device to a heat sink assembly comprising: supporting aheat sink assembly independently of a bare die microelectronic device;and, urging a surface of a bare die microelectronic device into directheat transfer relationship at a thermal interface with a heat sinkassembly under controlled forces.
 13. The method of claim 13 furthercomprising: placing a thermal interface material at the thermalinterface between the bare die microelectronic device and heat sinkassembly.
 14. The method of claim 12 wherein the urging includes a forceapplying mechanism that is actuatable to deliver the controlled forcesto the bare die microelectronic device to urge the latter into the heattransfer relationship with the heat sink assembly.
 15. The method ofclaim 14 further comprising supporting the bare die microelectronicdevice on a printed board through a surface mount assembly.
 16. Themethod of claim 15 wherein the force applying mechanism delivers thecontrolled forces to a surface of the printed board that is opposed to asurface upon which the bare die microelectronic device is mounted. 17.The method of claim 16 wherein the supporting the bare diemicroelectronic device through a surface mount assembly includes using afirst ball grid array assembly.
 18. The method of claim 17 furthercomprising providing a filler material in the ball grid array assemblythat serves to fill any air voids and adhesively attaches the first ballgrid array assembly to the printed board.
 19. The method of claim 18further comprising providing a filler material that minimizes oreliminates relaxation thereof following compression loading of the firstball grid array assembly.
 20. The method of claim 19 further comprisingproviding the bare die microelectronic device with a silicon bare diemounted on a dielectric member, a second ball grid array assemblymounting the silicon bare die to the dielectric member, and a fillermaterial in the second ball grid array assembly that attaches thesilicon bare die to the dielectric member and serves to fill air voidstherebetween and substantially reduces or eliminates relaxation thereoffollowing compression loading of the second ball grid array assembly.21. The method of claim 20 wherein supporting the heat sink assemblyincludes providing mounting fastening members that secure the heat sinkassembly to a supporting structure, and further supporting the heat sinkassembly with a plurality of resiliently deformable spacers interposedbetween the printed board and mounting fastening members.
 22. A computersystem comprising: a computer housing assembly; a printed board mountedin the computer housing assembly; a bare die microelectronic devicemounted on the printed board; a heat sink assembly, a heat sink mountingassembly for mounting the heat sink assembly on the computer housingassembly independently of the bare die microelectronic device; and, aforce applying mechanism that loads, under controlled forces, a surfaceof the bare die microelectronic device into a direct heat transferrelationship at a thermal interface with the heat sink assembly.